At the risk of sounding stupid, how can I tell which are current output and which are voltage?

Thanks, Mark.

By careful reading of the datasheet. But the term 'two wire' is kind of a process control world 'buzz word' for current loop devices. Most sensors used in process plants use current loop sensors because it allows for long cable runs between the sensors and the control system that won't be effected by the length of cabling causing voltage drops that would effect accuracy.